Baths used in igneous electrolysis



Dec. 1, 1959 E. J. B. PRUVOT 2,915,441

BATHS USED IN IGNEOUS ELECTROLYSIS Filed March 22, 1956 INVENTOR EMILEJEAN BAPT/5T E PRUVOT ATTORNEY nited States Patent f; 2,915,441 BATHSUSED INIGNEOUS ELECTROLYSIS Emile Jean BaptistePruvot,Saint-Jean-de-Maurienne, France, assignor to Pechiney, Compagniede Produits Chimiques et Electrometallnrgiques, Paris, France, acorporation of France Application March 22, 1956, Serial No. 573,139Claims priority, application France March 30, 1955 8 Claims. c1, 204-44The present invention is directed to improvements in the composition ofand method of use of chlorofluorinated baths employed inii'geneouselectrolysis for producing and refining .metalsand :moreparticularly,aluminum, beryllium, titanium and zirconium. t A further "and-particularobje'ct'oi the present invention is' an improved method forrefiningtitanium and zirconium'scrap'and an improved apparatus' forcarrying out this .method.

2,915,441 Patented Dec. 1, .1959

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inally present or ,produced by a secondary reaction, is

out according to the three-layer method: A layer of aheavy alloycontaining thesimpure metal to berefined and servingas anode, rests inthe liquid state on the bottom of thecrucible of the electrolysis .cell.Superposed .thereon, is -a layer of molten electrolyte, ontop of whichfloats the refined metal constituting thecathode.

Theel'ectrolyte is, generally composed'of a mixture of chlorides andfluorides containing an appreciable proportion of a halogenatedcompound: of the metal which it is desired to refine in order. the'avoidthe deposition,'at

the cathode, of alkali or alkaline-earth metals, the salts of whichgenerallycons'titute the major portion of the electrolyte. I

For example, in refining aluminum, there is frequently used a bathcontaining 6O% .BaCl 'and 40% chiolithe (2AlF 3NaF), a small amount ofsodium chloride being added to this mixture.

It has also been proposed to use a sodium-free bath containingbariumchloride,calcium fluoride and aluminum fluoride, the proportionof'the latter compound varying between 25% and In the course of theelectrolysis,.additions oflsalts are frequently made so .as to".maintain the composition of the bath within'thosedimits' which make itpossible to obtain the refined metal atthe cathode. '5 These additionscontain aluminum fluoride having a certain content of free alumina whichisalways present as a result of its method of production. The residualmoisture in this salt causes hydrolysis reactions of the general type:

During the operation, this alumina is precipitated on the anodeincreasing thereby the ohmic resistance at the contact of the twolayers; it islikewise" deposited all around the electrolysis crucibleand forms crusts heavily loaded with alumina, since the alumina contentmay reach 50%,"thusleading to a contraction in the" crosssection of thecrucible'and, hence,"to an increase in the current densityanda furthercause for the'increas'e. in the'voltageat the terminals'of'the cell.

For all of'these 'reasons,'fre'e' alumina, whether o'rig highlyundesirable.

The present invention, which is based on applicants investigations,relates to a process which makes it possible to avoid these drawbacks bycausing the disappearance of the oxidized compounds from theelectrolysis bath or the correcting bath. This process consists intreating the bath with a compound which is capable of exerting ontheoxidized compounds an energetic fiuorinatin It is action and oftransforming them into fluorides. possible to use gaseous hydrofluoricacid; however, good results are obtained by the use of alkaline oralkalineearth bifluorides, as for example NaF.HF, KF.HF, CaF .2l-IF.

According to a preferred form of the invention, there is added to theconstituents of the bath'ammonium bifluoride, which disappearscompletely during the course of the treatment.

The fluorinating action of ammonium bifluoride starts at 200 C., atemperature sufficiently high to dehydrate most of the halides of whichthe bath is constituted.

The action of the bifluoride may advantageously be extended up to 600 C.

The example set out below, which is solelygiven by way of illustration,and not limitation, will give a better understanding of the invention:

Example 1 In the electrolytic refiing of aluminum by means of a bathcontaining 60% barium chloride and 40% chiolithe (2AlF 3NaF), thecorrection of the bath is carried out most often by means of a mixtureof salts containing NaCl, BaCl and AlF in suitable proportions,sometimes in equal parts, and sometimes in the proportion ofby way ofexample25% NaCl, 35% BaCl 40% AlFg. v i

a If there be heated together grams BaCl containing 1.5% water 8 0gramsNaCl containing 1.4% water 80 grams All-' containing 9% water, and 6%free'Al O the. mixture melts at 600 C. but contains considerablequantities of insoluble alumina muds.

In contrast, if there be added to a mixture having the amount ofhydrofluoric acid and NH F vapor is evolved,v

there is obtained a perfectly fluid and transparent bath, withoutalumina. This bath solidifies around 500 C., can be cooled and thenremolten, which operation can be carried out four times in successionwithout formation of any'turbidity. The alumina, which is insoluble inthis bath, has therefore been completely transformed into aluminumfluoride and the moisture of the starting materials, which would producealumina by hydrolysis, has been eliminated.

It is advantageous to dispose a hood above the-furnace during themelting operation so as to recuperate the evolved vapors.

While the example set out above relates to the elimination of aluminafrom a correcting bath, the process can equally be applied to therefining bath itself before the cell is started.

The process which is the object of the present invention is not limitedto the refining of aluminum, but can also be applied in all cases wherethe presence of an oxidized compound is undesirable, both inelectrolytic production (elaboration) as well as in the refining ofmetals. I

As will be apparent from the foregoing, applicant has disclosed aprocess which enables the elimination of undesirable, oxidized compoundscontained in the chlorofiuorinated baths used in electrolysis. Torecapitulate: this process consists in treatingthe bath with anenergetic fluorinating agent, for example, hydrofluoric acid or ammoniumbifluoride, which is capable ofv transforming the oxidized compoundsinto fluorides.

However, in the practical application of the present invention, it hasbeen observed that in the course of the treatment certain chlorides maybe partially transformed into fluorides'which, occasionally, presentsdrawbacks.

Accordingly, a further feature of the present invention consists inexcluding from the treatment with the fluorinating agent, thosechlorides which are susceptible of being transformed into fluoridesundesirable in the electrolysis. These chlorides may be separatelydried, if necessary in a vacuum, before being mixed with the substanceswhich are subjected to the fluorinating treatment. The followingexample, merely given by way of illustration and not limitation, willexplain further this particular feature of applicants invention:

Example 2 In the continuous refining of aluminum by means of a bathcontaining 60% BaCl and 40% chiolithe (2AlF .3NaF), there areperiodically made corrective additions containing AIF BaCl and NaCl. Thefluorinating treatment by means of NH F.HF transforms a portion of thebarium chloride into barium fluoride, the presence of which changes thedensity, the fusion point and the selectivity of the bath which, as aresult of such changes, may be rendered unsuitable for the aluminumrefining operation. Accordingly, and following applicants furtherimprovement, the procedure adopted is as follows:

parts of aluminum fluoride are carefully mixed with 3 parts of ammoniumbifluoride, NH F.HF. This is heated between 400 and 500 C. with constantstirring and while proceeding very carefully and slowly at the beginningin order to avoid vaporization (entrainment). The end of the reaction isindicated by cessation of bub bling. The heating is preferably carriedout in a graphite crucible, proceeding with successive additions of themixture to be heated in order to avoid violent evaporation(entrainment). An aluminum stirrer can be used.

When the bubbling has ceased, the hot aluminum fluoride is poured intoan aluminum basin and permitted to cool. 'As soon as it is cold, it isimmediately mixed with the two other correcting salts BaCl and NaClwhich previously have been carefully dried at 120 C. There is obtainedin this manner an addition bath free of oxides.

and capable of giving excellent results in the refining of aluminum.

The present invention finds a particularly important application in theelectrolytic refining of titanium and zirconium. The metallurgy oftitanium as well as of zirconium produces a metal which is frequentlycontaminated with impurities, as for example, oxygen and nitrogen. As aresult, an appreciable percentage of the resultant metal, sometimesbetween 40% and 50%, does not possess the necessary ductility andconstitutes scrap. The production cost of ductile titanium and zirconiumis appreciably increased as a result of this circumstance and it becomesmandatory to treat this scrap. It is afurther object of the presentinvention to use the improved process disclosed herein in the treatmentof such titanium and zirconium scrap.

Electrolytic refining processes of metals have been known for a longtime: The crude metal is used as an anode which is dissolved little bylittle by the anions of the electrolyte and is deposited in a pure stateonthe cathode. This process is especially used in refining aluminum andberyllium, using as electrolytes molten alkaline vor alkalineeearthhalides containing ac'ertain proportion of the halide of the metalswhich it is desired to refine, so as to maintain in the electrolyte acertain concentration of cation of the metal to be deposited, forexample, between 35 and 115 gr. of this metal per kgm. of bath.

As a special feature of this invention, applicant has applied thepresent invention to improve this general refining method, which is alsoapplicable to the refining of titanium and zirconium.

In accordance with the present invention, there is manufactured ductiletitanium and Zirconium having a Brinell hardness less than 120, andfrequently less than 100, by the electrolytic refining of scrap metal ina bath preferably composed of molten alkali chlorides; the necessaryquantity of titanium or zirconium ions is obtained preliminarily byacting with a tetrachloride, TiCL, or ZrCl on titanium or zirconiumscrap, which constitutes the anode element of the refining cell. Animportant feature of the improvedprocess according to the invention,consists in eliminating all trace of oxygen in the electrolyte prior tostarting the refining operation; indeed, it has been observed that whensuch elimination is suitably carried out, there is obtained at thebeginning of the refining operation metallic titanium the Brinellhardness of which is less than 100. A similar improvement has been foundin the case of the zirconium also. i

In a preferred embodiment of the invention, there is used as deoxidizingagent ammonium bifluoride, NH F.HF. To this end, the procedure is asfollows:

NaCl or NaCl+KCl are mixed in the dry condition, with for example, 10%of NH F.HF. The mixture is heated to between 300 and 400 C. to start thedeoxi dizing effect. A small quantity of titanium or zirconium chips ofthe order of a few percent of the bath is added, in order to reduce thesulphates into sulfides; thereupon, the chlorides are molten at amaximum temperature of 900 C. Under these conditions, the sulfides aredestroyed by the residual bifluoride which completes in a thoroughmanner the deoxidization and de-sulfurization. This operation can becarried out in air but, preferably, is carried out in an argonatmosphere as is also the subsequent electrolysis.

During the deoxidizing operation, a portion of the alkali chlorides istransformed into fluorides which do not in any fashion interfere withthe refining operation.

The titanium or zirconium tetrachloride is then injected into theinterior of abasket containing the scrap of the metal to be refined, andwhich will subsequently form the anode assembly during the electrolysis.In the presence of a large excess of metal, the TiCl or ZrCl; istransformed almost completely. into bichlorides.

The injection of the tetrachloride can be stopped when the bath containsabout 40 grms. of titanium or zirconium per kgm. of electrolyte, but theinjection can be contined until there is obtained, for example, 60 togrms. of the metal to be refined per kgm.

The electrolytic refining is thereafter carried out in known mannerwithout it being necessary to reinject the tetrachloride.

At the cathode, there are obtained needles of titanium or zirconiumwhich may attain 4 centimeters in length and a cross-section ofseveral-square millimeters. The portion of this deposit which does notpass through a 100 mesh screen attains or exceeds 95 The metal yieldfrequently attains The following example will give an idea of the purityof the produced metal:

Parts per million Oxygen Very light traces Titanium thus prepared, whentested with a 10 mm. ball and a pressure of 3 tons has a Brinellhardness varying between 130 and 73, with an average of 100. Obviously,this constitutes an excellent ductile titanium.

The present invention also comprehends an electrolysis cell whichenables the refining of titanium or zirconium to be carried out in acontinuous manner using the resistance of the bath to maintain theelectrolyte in molten condition without the use of an external heatsupply.

Figures 1 and 2 show an embodiment of an electrolysis cell in accordancewith the invention, wherein Figure 1 shows a vertical sectional viewalong the line 1-1 of Figure 2, of the electrolysis cell, and

Figure 2 is a plan View of the cell partly in section, the section planebeing along line 22 of Figure 1.

In these figures; 1 designates the external metallic casing of the cell,2 the heat insulation jacket, 3 the interior casing which may be ofsheet iron. The space between the casings 1 and 3, filled with heatinsulation bricks, constitutes a tight assembly in which a vacuum can bemade through connection 4. 5 is a circular iron basket made, forexample, of a sufliciently close metallic screen; this basket, filledwith titanium or zirconium scrap, is connected at 6 to the positivepole.

The cathodes 7 are formed of metallic tubes which are closed at theirlower ends; they dip into the molten electrolyte the level of which isrepresented by the line 8-8. These cathodes are extended upwardly bytubes 9, 9 of a smaller diameter, which makes it possible to remove themwhen the titanium or zirconium deposit has become too thick; these tubesare connected at their upper ends to the negative pole by the conductor16.

The muds accumulate at the bottom of the cell at 11, and can beperiodically removed through a lock chamber 12 closed with a tight cover13.

A cylindrical space or chamber 14 extends above the electrolysis cell;it is lined at its circumference by the basket 5 containing the titaniumor zirconium scrap and is traversed by the rods 9 carrying the cathodes.The cylindrical chamber is fixed at its lower end to the electrolysiscell by means of a tight joint 15.

To the upper portion of the cylindrical chamber 14, there is fixed by atight joint 16 an assembly of cylindrical lock chambers 17 (one percathode) which can be closed near their lower ends by valves 18; theheight of the lock chambers above the valves is suflicient so that thecathodes 9 can be withdrawn therein. At the upper part of each lockchamber, there is disposed a vacuum and argon connection 19.

Openings 20 enable the introduction, when necessary, of an iron bar tofacilitate the descent of metal scrap to be refined into the basket 5,or of a tube for introducing TiCl or ZrCl to form the titanium orzirconium cations before the start of the electrolysis.

The quantity of titanium or zirconium scrap contained in the basketpermits continuous operation of the electrolysis for a period of todays, but there are provided three scrap feeding lock chambers 21disposed above the iron ring of the cover of the cylindrical chamber 14,at the side of the openings 20. By'the use of these lock chambers, thescrap metal to be formed falls directly into the basket 5.

As the electrolysis progresses, it is necessary to remove and change thecathodes one after the other; too thick a deposit of refined metalprevents normal operation of the electrolysis due to the risk of shortcircuit between anode and cathode. The cathode, which it is desired toremove, is first lifted into the cylindrical chamber in order that itmay drain; it is then introduced into its lock chamber 17, and the valve18 thereof is closed; it cools therein and the lock chamber can then beopened at its upper end for the removal of the cathode coated withtitanium or zirconium, and for the replacement of the cathode.

Following closure of the lock chamber, ,a' vacuum is made therein; it isthen filled with argon; the valve 18 is then opened, and the new cathodeis lowered into the molten electrolyte.

The same lock chambers which serve for the transfer of the cathodes canadvantageously be used for the introduction of the bath previouslytreated asset out above.

The electrolysis cell in accordance with this invention makes itpossible to obtain a continuous refining operation. The compact form ofthe cell makes it possible to maintain the electrolyte in a liquid stateby the heat evolved by the passage of the electric current therethrough.The cell can operate with current strengths of thousands of amperes.

While in the preferred method of refining zirconium and titaniumaccording to this invention use is made of molten alkali chlorides, itis understood that other halides of alkali metals, as well as halides ofalkaline earth metals can be employed, such as bromides, iodides andfluorides of alkaline and alkaline earth metals and magnesium.

The term preparation as used in the appended claims covers both theproduction of metals, as by reduction of their compounds, as well as therefining of impure metals.

I claim:

1. In the preparation of metals selected from the group consisting ofaluminum, beryllium, titanium, and zirconium, by the igneouselectrolysis of a bath comprising a mixture of halides and a halide ofthe metal to be prepared, said metal halide being contaminated with anoxide of said metal, the improvement of inhibiting the deposition ofsaid oxide in the cell which consists in: treating the bath with anagent capable of exerting a fluorinating action on said oxide and oftransforming it into a fluoride whereby the bath is maintained in asubstantially fluid and transparent condition and substantially free ofthe metallic oxide.

2. Process according to claim 1, wherein the fluorinating agent is acompound selected from the class consisting of gaseous hydrofluoricacid, ammonium bifluon'de, alkali metal bifluorides and alkaline earthmetal bifluorides.

3. Process according to claim 1, wherein the fluorinating agent isammonium bifluoride which is reacted with the bath at a temperaturewithin the range of 200600 C.

4. Process according to claim 1, wherein the halide constituents of thebath contain residual moisture.

5. Process according to claim 1 wherein the mixture of halides consistsof alkali and alkaline earth metal chlorides and fluorides.

6. In the preparation of metals selected from the group consisting ofaluminum, beryllium, titanium and zirconium, by the igneous electrolysisof a bath comprising a mixture of metallic chlorides and fluoridesincluding that of the metal to be prepared, said fluorides beingcontaminated with a metallic oxide, the improvement of inhibiting thedeposition of said oxide in the cell which consists in: treating thefluoride constituents of the mixture at an elevated temperature with afluorinating agent selected from the class consisting of gaseoushydrofluoric acid, ammonium bifluoride, alkali metal bifluorides andalkaline earth metal bifluorides, whereby the oxide is transformed intoa fluoride, cooling the so-treated metal fluoride, drying the chlorideconstituents of the bath, mixing the dried chlorides with the cooledmetal fluoride and using the resultant mixture as an additive for theelectrolysis bath.

7. In an electrolytic process for treating scrap of a metal selectedfrom the group consisting of titanium and zirconium to produce a refinedductile product, the steps comprising: heating at least one halide ofmetal selected from the class consisting of: alkali and alkaline earthmetals, and wherein said halide is contaminated with oxygen-containingcompounds of the metal to be prepared;

contacting the heated halide with a bifluoride and further heating thehalide to a, temperature sufiicient to melt the same, whereby theoxygen-containing compounds react with the bifluoride and aredeoxidized; contacting scrap of the selected metal with the moltendeoxidized halide, and injecting a tetrachloride of the selected metalinto the scrap, thereby forming bichloride of the selected metal. a

8. Process according to claim 7, characterized in that a mixture ofsodium and potassium chlorides is treated with ammonium bifiuoride andheated to a temperature 0f.'300-400 CL, comminuted metal to be refinedis added to the heated chloride mixture, and the heating is continued toa temperature not in excess of 900 C. to melt the chlorides.

References Cited in the file of this patent UNITED STATES PATENTS578,633 Gooch Mar. 9, 1897 736,020 Roepper Aug. 11, 1903 786,244Blackmore Mar. 28, 1905 842,256 Seward et a1. Jan. 29, 1907 2,451,494Johnson 'Oct. 19, 1948 2,731,402 Topinka et a1. Jan. 17, 1956 102,755,240 Normore et al. July 17, 1956 FOREIGN PATENTS 713,446 GreatBritain Aug. 11, 1954 OTHER REFERENCES The Production of Zirconium byFused Salt Elev trolysis, AEC Report NYC 3117, J an. 1, 1952.

1. IN THE PREPARATION OF METALS SELECTED FROM THE GROUP CONSISTING OFALUMINIUM, BERYLLIUM TITANIUM, AND ZIRCONIUM, BY THE IGNEOUSELECTROLYSIS OF A BATH COMPRISING A MIXTURE OF HALIDE BEING CONTAMINATEDWITH AN PREPARED SAID METAL HALIDE BEING CONTAMINATED WITH AN OXIDE OFSAID METAL, THE IMPROVMENT OF INHIBITING THE DEPOSITION OF SAID OXIDE INTHE CELL WHICH CONSISTS IN: TREATING THE BATH WITH AN AGENT CAPABLE OFEXERTING A FLUORINATING ACTION ON SAID OXIDE AND OF TRANSFORMING IT INTOA FLUORIDE WHEREBY THE BATH IS MAINTAINED IN A SUBSTANTIALLY FLUID ANDTRANSPARENT CONDITION AND SUBSTANTUALLY FREE OF THE METALLIC OXIDE.